Muscle wasting, which occurs mainly by an activation of the ubiquitin- proteasome degradative pathway, is a prominent, debilitating feature of many disease states, including diabetes mellitus and renal failure. Recently, using a newly established cell-free system, we have been able to demonstrate that rates of ubiquitin (Ub) conjugation increase in atrophying muscles from septic; tumor-bearing, diabetic and uremic rats, and that a subset of Ub conjugating enzymes, the N-end rule pathway, is responsible for most of the enhanced Ub conjunction in these atrophying muscles. This is an interesting, unexpected discovery because the N-end rule pathway has been viewed as a minor ubiquitination system that was only involved in the elimination of certain abnormal polypeptides. These results raise the possibility that in cachexia, muscle proteins may be modified to become substrates for this pathway. We propose to use our newly developed cell-free system to further characterize this process. We will measure the abundance and activity of the N-end rule pathway enzymes (E1, E2/14K, and E3alpha) to identify the ones which are responsible for the enhanced proteolysis, and identify the substrates in muscle for these enzymes. In collaborative studies, we will genetically produce animals in which these enzymes are deleted to directly show their requirement in muscle atrophy. Finally, since most of the loss of muscle protein during muscle atrophy is from myofibrillar components, we will begin to study how the myofibril may serve as a source of substrates of the Ub-proteasome pathway by developing an assay for myofibril disassembly. Defining the components of the Ub-proteasome pathway and myofibril disassembly which are modulated in diabetes and renal failure should not only help to illuminate the regulation of muscle protein turnover, but also may allow the development of inhibitors that could combat the morbidity of these catabolic diseases. These studies will be performed in the laboratory of Dr. Alfred Goldberg, a leader in the fields of muscle proteolysis and the Ub- proteasome pathway. The applicant is a graduate of the M.D./Ph.D. program at UCLA, completing a Nephrology fellowship at the Beth Israel Deaconess Medical Center and Harvard Medical School. His long-term goal is to develop a research program centered on problems of protein folding and degradation relevant to kidney disease. This proposal offers the unique opportunity for the applicant to obtain further cell biology training, gaining experience in animal physiology, DNA technology, and biochemistry, while studying clinically relevant problems in renal disease.